LU-5130 lod: Do not merge attr for striped directory

Does not need to merge attributes in lod_attr_get(),
because client will merge attributes itself for striped
directory (see lmv_merge_attr()), and also there are no
logic in MDD will reply on directory nlink/size/time.

Besides lod_attr_get is in the path of object initialization,
mdd_object_start()->mdd_la_get()->lod_attr_get(), if it tries
to get attribute from other MDTs, and if other MDTs are not
ready yet, it will cause unnecessary failures for object
initialization.

Change-Id: I56395a40fcd82142f99228a0776b5b7338c633ac
Signed-off-by: Wang Di <di.wang@intel.com>
Reviewed-on: http://review.whamcloud.com/10841
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Fan Yong <fan.yong@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>